1. Field of the Invention
The present invention relates to a ferrule assembly for connection with an optical part incorporating an optical fiber, and particularly to a receptacle-type optical transmission module using a ferrule assembly.
2. Description of the Related Art
In recent information communication field, increase of information amount is followed by a need to transmit a large quantity of information at high speed, and high-speed large-capacity optical communication networks such as optical subscriber systems are now spreading. For popularization of the optical subscriber systems, cost-down of optical modules is an unavoidable task. The cost of optical modules consists of parts cost, assembly cost, and test and adjustment cost; among others, the assembly cost and adjustment cost constitute a major part of the total cost. As a means of reducing the assembly cost, receptacle-type optical modules have come to be spotlighted in recent years.
Parts mounted on a printed wiring board to be incorporated in a communication device are generally classified into surface mounting type and through-hole mounting type. A representative example of the surface mounting type parts is LSI, which is so-called flat package type in shape. This type of parts are soldered by a technique called reflow soldering. Namely, a pasty solder is printed on a printed wiring board, a surface mounting type part is adhered to the pasty solder, and soldering is carried out in a conveyor furnace in which the solder surface temperature is brought to 220xc2x0 C. or higher.
Representative examples of the through-hole mounting type parts are high-capacity capacitors and LSIs with a large number of terminals (200 terminals or more). The LSIs with a large number of terminals have a terminal form called PGA (Pin Grid Array). These through-hole mounting type parts are soldered by a technique called flow soldering. Namely, terminals of the through-hole mounting type part are inserted in through-holes of a printed wiring board, and soldering is carried out by placing the opposite side of the part mounting surface of the printed wiring board in a solder bath at a temperature of about 260xc2x0 C.
For a technique of mounting optical modules on a printed wiring board by soldering just like the surface mounting type parts and the through-hole mounting type parts, the so-called pig-tail type optical modules with an optical fiber cord are unsuitable. Normally, an optical fiber cord has a nylon covering, which has a heat resistance of only about 80xc2x0 C. and, therefore, will melt in a soldering step. In addition, the optical fiber cord itself causes troubles or difficulties in containing and handling at production sites, thereby conspicuously lowering the efficiency of mounting on printed wiring boards.
Therefore, in order to enable soldering of optical modules and contrive a reduction in production cost, it is essentially required to provide a so-called receptacle-type optical module which does not comprise an optical fiber cord. The receptacle-type optical module is an optical module having a connector portion so that an optical fiber cord having a covering with low heat resistance can be fitted to and detached from the module. The receptacle-type optical module allows application of a solder reflow step at high temperature and, therefore, can be mounted on a printed wiring board simultaneously with surface mounting type electronic parts.
By simultaneously mounting optical parts and electronic parts on a print wiring board, a plurality of mounting steps using solders having different melting points required in the prior art can be simplified, which promises a large reduction of cost. As a means of reducing adjustment cost, a self-alignment technique has been investigated. In the technique, guide portions specified in positional relationship with optical parts at high precision are formed on the optical part mounting substrate, and optical wave guide parts (optical fiber, ferrule, etc.) are mounted along the guides, thereby accomplishing optical coupling. The self-alignment technique makes it possible to omit alignment steps which require a large number of steps and high cost.
Japanese Patent Laid-open No. 11-264926 (1999) discloses a structure of an interface portion between a PLC (Planar Lightwave Circuit) and an optical fiber. The optical module described in the publication has a structure such that a ferrule incorporating an optical fiber is mounted along a groove formed in an optical device mounting substrate. This structure is characterized in that optical coupling can be achieved automatically (self-alignment) by precisely designing the relative positions of the groove and the optical part.
An optical module disclosed in Japanese Patent Laid-open No. 7-318764 (1995) proposes another mode of self-alignment in which a bare optical fiber is protruded from an end of a ferrule and the optical fiber is inserted and fixed in a guide groove formed in an optical device mounting substrate. This structure is characterized in that positioning of the optical fiber can be carried out directly by the guide groove and, therefore, a high mounting precision can be obtained.
As a further example of the prior art, Japanese Patent Laid-open No. 2000-171668 may be mentioned. An optical module described in this publication proposes a further mode of self-alignment in which a bare optical fiber is semi-exposed by cutting away a part of a ferrule and is mounted and fixed in a guide groove formed in an optical device mounting substrate. The optical module of this structure is characterized in that since a part of the ferrule is used for retaining the bare optical fiber, a retaining part can be omitted and adhesion area can be made small.
There are four characteristics required of an inexpensive and highly reliable receptacle-type optical module: (1) easy mounting, (2) high strength, (3) capability of end face polishing and measure against reflection, and (4) high positioning precision. Particularly, the end face polishing is expected much as a means of coping with reflection. Reduction of reflected-back light has become a major task in recent years, and in order to reduce reflected-back light from a value of about 25 dB at present to a value of not less than 40 dB required in future, a slant polishing structure in which a fiber end face is slanted against optical axis is effective. For realizing this structure, it is required that the fiber end can be polished.
The optical module described in the above-mentioned Japanese Patent Laid-open No. 11-264926 (1999) has the following problems. It is required to form a guide groove having a depth corresponding to the radius of the ferrule (about 650 xcexcm in the case of an MU-type ferrule of standard specifications), so that etching time is increased and it is difficult to provide a protective film. In addition, a large sized optical part mounting substrate is required. The large guide groove leads to low controllability of width at the time of formation thereof, and long-time etching may easily produce an abnormal etched surface at a slant surface. Further, since positioning is carried out using the outer shape of the ferrule, a loss in optical coupling will be generated due to eccentricity of the inside diameter of the ferrule and eccentricity at the time of insertion of optical fiber.
The optical module disclosed in the Japanese Patent Laid-open No. 7-318764 (1995) has the following problems. It is impossible to polish end faces of the optical fiber. In addition, since the optical fiber is entirely exposed from the ferrule, there is a high danger of breaking of the optical fiber at the time of handling. Further, since an optical fiber fixing portion and a ferrule fixing portion are independent in the optical module, a stress may be generated at the boundary portion between the optical fiber and the ferrule, possibly producing bad effects on reliability.
The optical module disclosed in the Japanese Patent Laid-open No. 2000-171668 has the following problems. End face polishing is impossible because there is a high danger of breaking of fiber. Besides, since the bare optical fiber cannot be fixed to the ferrule at the cutaway flat portion, there is the danger of breaking of the optical fiber.
Accordingly, it is an object of the present invention to provide a receptacle-type optical module suitable for reductions in cost and size.
It is another object of the present invention to provide a ferrule assembly required for assembling a receptacle-type optical module.
In accordance with an aspect of the present invention, there is provided a ferrule assembly comprising: a ferrule having a through-hole, a first end portion, a second end portion on the opposite side of the first end portion, and an intermediate portion between the first end portion and the second end portion; and an optical fiber inserted and fixed in the through-hole; wherein the ferrule has at the intermediate portion a cutaway flat portion allowing the optical fiber inserted in the through-hole to be semi-exposed, and the optical fiber has its entire circumference held by the ferrule at least at the first and second end portions of the ferrule.
Preferably, the ferrule has a cut worked portion at the first end portion and/or the second end portion. According to the ferrule assembly of the present invention, it is possible to realize all of the above-mentioned four requirements, which cannot be fulfilled in the prior art.
(1) Since the entire circumference of the optical fiber is fixed to the ferrule at both ends of the ferrule, the ferrule assembly can be handled like an MU-type ferrule of standard specifications, and mounting thereof is easy.
(2) Because the entire circumference of the optical fiber is fixed to the ferrule at both ends of the ferrule, there is no danger of breaking of the optical fiber.
(3) End faces of the ferrule and the optical fiber can be polished.
(4) High-precision self-alignment by a bare optical fiber can be achieved.
In accordance with another aspect of the present invention, there is provided a block type ferrule assembly comprising: a block having a through-hole, a first end portion, a second end portion on the opposite side of the first end portion, and an intermediate portion between the first end portion and the second end portion; an optical fiber inserted and fixed in the through-hole; and a plurality of guide pins fixed to the first end portion of the block, wherein the block has at the intermediate portion a cutaway flat portion allowing the optical fiber inserted in the through-hole to be semi-exposed, and the optical fiber has its entire circumference held by the block at the first and second end portions of the block.
In accordance with a further aspect of the present invention, there is provided an optical module comprising: a substrate having a first groove and a second groove orthogonal to the first groove; an optical wave guide structure formed on the substrate, comprising an optical wave guide core portion having its one end aligned with one end of the first groove, and an optical wave guide clad portion covering the optical wave guide core portion; a ferrule having a through-hole, a first end portion, a second end portion on the opposite side of the first end portion, and an intermediate portion between the first and second end portions; and an optical fiber inserted and fixed in the through-hole, wherein the ferrule has at the intermediate portion a cutaway flat portion allowing the optical fiber inserted in the through-hole to be semi-exposed, the optical fiber has its entire circumference held by the ferrule at least at the first and second end portions, and the ferrule is fixed to the substrate at the cutaway flat portion so that the first end portion of the ferrule is inserted in the second groove, and the optical fiber is inserted in the first groove of the substrate with one end of the optical fiber abutted on one end of the optical wave guide core portion.
In accordance with a still further aspect of the present invention, there is provided an optical module comprising: a first substrate having a first groove and a second groove orthogonal to the first groove; a second substrate having a third groove and a fourth groove orthogonal to the third groove; a first optical wave guide structure formed on the first substrate, comprising a first optical wave guide core portion having its one end aligned with one end of the first groove, and a first optical wave guide clad portion covering the first optical wave guide core portion; a second optical wave guide structure formed on the second substrate, comprising a second optical wave guide core portion having its one end aligned with one end of the third groove, and a second optical wave guide clad portion covering the second optical wave guide core portion; a ferrule having a through-hole, a first end portion, and a second end portion on the opposite side of the first end portion; and an optical fiber inserted and fixed in the through-hole, wherein the ferrule has first and second cutaway flat portion allowing the optical fiber inserted in the through-hole to be semi-exposed, the optical fiber has its entire circumference held by the ferrule at least at the first and second end portions of the ferrule, and the ferrule is fixed to the first and second substrates respectively at the first and second cutaway flat portions so that the first end portion of the ferrule is inserted in the second groove of the first substrate, while the second end portion of the ferrule is inserted in the fourth groove of the second substrate, and the optical fiber is inserted in the first and third grooves of the first and second substrates, with one end of the optical fiber abutted on one end of the first optical wave guide core portion and with the other end of the optical fiber abutted on one end of the second optical wave guide core portion.
In accordance with a still further aspect of the present invention, there is provided an optical module comprising: a substrate with a first marker formed on its surface; an optical wave guide structure formed on the substrate, comprising an optical wave guide core portion, and an optical wave guide clad portion having a narrow first portion and a wide second portion each covering the optical wave guide core portion; a glass plate having a wide first groove, a narrow second groove, a third groove formed between the first and second grooves orthogonally to the first and second grooves and a second marker, said glass plate being fixed on the substrate so that the second marker is positioned to the first marker and the first groove contains the first portion of the optical wave guide clad portion; and a ferrule assembly including a ferrule having a through-hole, a first end portion, a second end portion, and an intermediate portion between the first and second end portions, and an optical fiber inserted and fixed in the through-hole, the ferrule having at the intermediate portion a cutaway flat portion allowing the optical fiber inserted in the through-hole to be semi-exposed, and the optical fiber having its entire circumference held by the ferrule at least at the first and second end portions of the ferrule, said ferrule assembly being fixed to the glass plate at the cutaway flat portion so that the first end portion of the ferrule is inserted in the third groove of the glass plate and the optical fiber is inserted in the second groove.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.